sub nearest {
   local $_ = 0+$_[0];
   return 0 unless $_;

   my $f = $_ <=> 0;
   $_ = abs($_);

   while ($_ >= 10) { $_ /= 10; $f *= 10; }
   while ($_ <   1) { $_ *= 10; $f /= 10; }

   return int($_ + 0.5) * $f;
}


sub up_nearest {
   local $_ = 0+$_[0];
   return 0 unless $_;

   my $f = $_ <=> 0;
   $_ = abs($_);

   while ($_ >= 10) { $_ /= 10; $f *= 10; }
   while ($_ <   1) { $_ *= 10; $f /= 10; }

   return (int($_) == $_ ? $_ : (int($_) + 1)) * $f;
}


foreach (
  0,
  5,
  10,
  50,
  0.1,

  5.1,
  11,
  51,
  0.11,

  5.5,
  15,
  55,
  0.15,

  5.9,
  19,
  59,
  0.19,
) {
   printf("%12.6f: %4s\n",  $_,    nearest( $_));
   printf("%12.6f| %4s\n",  $_, up_nearest( $_));
   printf("%12.6f: %4s\n", -$_,    nearest(-$_)) if $_;
   printf("%12.6f| %4s\n", -$_, up_nearest(-$_)) if $_;
}